1. Field of the Invention
The present invention relates to the coating of objects by the electrostatic deposition of particles thereon and more particularly pertains to a grid for establishing an electrostatic field of high intensity and controlled geometry.
2. Description of the Prior Art
It is well known in the art to utilize an electrostatic fluidized bed for the electrostatic deposition of particles on a variety of objects. In operation powdered plastic coating material from a conventional fluidized bed reservoir is given an electrostatic charge by suitably arranged corona discharges, and is attracted to an appropriately biased metallic substrate. The powder-coated substrates are subsequently heated or otherwise treated so as to cause the plastic powder to flow into a dense coating layer and to cure.
While electrostatic fluidized beds in general have been satisfactorily used to coat a variety of objects, providing relatively uniform coatings on substrates of complex shape, difficulties have been encountered in achieving this uniformity. This is so because the charged particle motions are determined by the electric field which exists in the space between the fluidized bed and the substrate, and by gravity, with the consequence that most of the particles which pass successfully to the substrate do so in a rather direct route, with the charged particles being deposited upon the most accessible area of effectively opposite charge, such that the surfaces of the substrate at different distances from the bed acquire heavy deposits upon their portions closest to the bed while developing progressively thinner coatings on their outer surfaces.
One such method and apparatus is disclosed for example in United States Letters Pat. No. 3,348,243, in the name of J. C. Barford, et al. Electrostatic charging of the fluidized bed particles is accomplished by providing a series of pointed electrodes positioned within the fluidized bed. However, the above mentioned difficulties of uniformity of coating were encountered. Accordingly, methods such as the simultaneous use of a plurality of isolated electrostatic fluidized bed sources appropriately positioned surrounding a suitably charged substrate of complex shape are disclosed.
A United States Letters Pat. No. 3,336,903, in the name of M. A. R. Point, acknowledges that difficulties are encountered in providing adequate control over the spatial extent of the electrostatically charged particles when using electrostatic coating equipment of the Barford, et al type and that the cloud of finely divided coating material to be applied to the surface of the substrate exhibited a tendency to wander. Point endeavored to eliminate this problem by providing auxiliary electrodes to keep the charged particles from wandering.
United States Letters Pat. No. 3,670,699, in the name of Gerald P. Sargent, sought to endeavor to solve the problem of uniform coating by replacing the series of pointed electrodes of Barford, et al with a web of moderately conductive particles or fibers which were incorporated into the porous membrane of an otherwise conventional fluidized bed.
United States Letters Pat. No. 3,828,729, in the name of William C. Goodridge, is exemplary of still a further attempt to solve the uniform coating problem. According to Goodridge, shields are utilized at various points to redistribute the charged particles in a more uniform manner.
However, it is clear that throughout the years substantial difficulties have been encountered in achieving relatively uniform coatings on substrates and to date such difficulties are still being encountered.